Although it is often believed that quartz crystal oscillators are very accurate, temperature changes and crystal aging cause substantial shifts in a crystal's natural frequency. For many systems, this frequency shift is unacceptable and some means of correction is needed.
For frequency correction, trimmer capacitors are inserted to fine tune the crystal frequency. If these are temperature compensating capacitors, the oscillator's frequency can be made more consistent over wide temperature swings. Disadvantages, however, include the need to tune these capacitors for each crystal, and of course the added cost of the components. Additionally, the capacitors provide no automatic correction for frequency shifts from aging.
Some devices, like ABB ERNIs which are known in the art, incorporate a timer interrupt mechanism that executes an interrupt after a counter attains a specific threshold value. The frequency this counter is incremented is related to the crystal, so an inaccurate crystal will cause inaccurate timer interrupt intervals. Since these intervals may be used in time critical functions such as a time stamping scheme as disclosed in U.S. patent application Ser. No. 414,999 entitled "A System for Time Stamping Events Using a Remote Master Clock", filed Sep. 29, 1989, it is essential that they be as accurate as possible.